In the automation of process engineering procedures, microcontroller-based measuring, open-loop and closed-loop control devices are employed. For economical reasons, an increasing importance must be attached to the automated, optimal adaptation to the process, apart from the base function according to its purpose. A widely-used closed-loop control device for process engineering procedures is the electro-pneumatic positioner for the continuous influence of fluid flows by means of a process valve or a regulating valve. This positioner is generally integrated in a superordinate process control loop in the form of a cascade and takes the role of the secondary auxiliary controller. The superposed process controller controls the secondary positioner, which in turn influences the process to be controlled by means of the regulating valve. The feedback of the actual process value using the process sensor closes the superposed process control loop. For different reasons, a nonlinear operating characteristic often occurs in the process to be controlled, which leads to a reduction in the control accuracy when a mostly linear PID-process controller is used. An improvement can be achieved by linearizing the operating characteristic by means of a correction characteristic. Firmly predetermined characteristic forms (equal percentage 1:25, 1:50, . . . ) or forms of characteristics which can be parameterized by value pairs are provided as correction characteristics. In the last case, the manual input of value pairs is realized. However, the drawbacks of the known methods consist in that the linearization takes place only in an approximate manner on the basis of predetermined forms of characteristics, or in that a complex manual determination of a free correction characteristic is necessary. This in turn results in a reduced control accuracy due to the insufficient or missing linearization. Furthermore, in case a wrongly dimensioned process valve is used in the control loop, the process control loop is only apparently optimized during the linearization, though no optimal result can be achieved.
Thus, a device and a method are provided for an open-loop or closed-loop control of a process, which permits compensation for nonlinearities.